The diagnostics and monitoring of structures, such as that carried out in the structural health monitoring field, are often accomplished by employing arrays of sensing elements coupled to a structure. These sensing elements then either passively monitor the structure for the presence of stress waves propagating therein, or actively interrogate the structure by generating such stress waves, where the stress waves have predetermined waveforms and amplitudes.
While these approaches can be effective in detecting damage, they also have their limitations. For example, damage detection is usually accomplished by comparing signals from the sensing elements with stored baseline waveforms indicative of some previous, or baseline, state of the structure. Difference between the baseline waveforms and the newer signals indicate that the structure has changed between the time of the baseline waveform and the time of the newer signals. However, this approach typically requires one to compensate for changes in the structure or environment that may be benign, yet also result in changes from the baseline waveform. For instance, temperature changes, differences in mechanical loading, or other conditions such as different external lubrication, scrap materials or debris, etc. can all potentially affect the sensor signals. Accounting for these and other effects can be difficult and unreliable, when it can be done at all.